**1. Introduction**

One of the prospective avenues of surface engineering is the deposition of hard, wear-, and oxidation-resistant transitional-metal based coatings for high-performance metal machining instruments by arc-evaporation and magnetron sputtering of composite targets [1–5]. Additionally, such coatings can be employed for heavy-duty friction pairs, high-temperature sensors, and resistive elements, critical parts for the aerospace industry. Tantalum disilicide (TaSi2) is often the material of choice for the engineering of coatings thanks to its high hardness (10–13 GPa) and refractoriness (2200 ◦C), low thermal expansion coefficient (8.8 <sup>×</sup> 10−<sup>6</sup> ◦C−1), high strength at temperatures above 1000 ◦C, and oxidation resistance (up to 1700 ◦C) [6–8].

TaSi2-based coatings can be produced by chemical vapor deposition and subsequent diffusion saturation [9,10], plasma spraying [11], sintering [12], slurry impregnation [13], electron beam evaporation [14], low-pressure chemical vapor deposition [15], diode or high-frequency sputtering [16], magnetron sputtering [17,18], and double-cathode sputtering [19]. In the case of surface engineering of metal-machining tools, chemical (CVD) and physical vapor deposition (PVD) techniques outperform other methods owing to the unrivaled capability to retain blade geometry, high quality, and homogeneity of the coating, as well as the general flexibility and possibility to fine-tune the properties of coatings.

The functional properties of TaSi2-based coatings can be enhanced by alloying. The addition of Al increases the adhesion strength of TaSi2 coatings by ~28%, despite an 8% decrease in hardness and a ~12% decrease in elastic modulus in [19,20]. Additionally, Al decreases the corrosion current density in Ta(Si1-xAlx)2 coatings from 7.08 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 3.05 <sup>×</sup> <sup>10</sup>−<sup>6</sup> <sup>A</sup>·cm−<sup>2</sup> [20]. Alloying of Ta-Si coatings by Zr and Ti improved the coatings' hardness (12–16 GPa) and corrosion resistance [21], whereas alloying by Ni produced amorphous coatings stable up to 900 ◦C for microelectronic applications [22]. Alloying by nitrogen and carbon is another promising avenue for enhancing the properties of TaSi2-based coatings. Such coatings were deposited by magnetron sputtering of composite TaSi2-SiC targets in the atmosphere with varied nitrogen content, resulting in hardness up to 26 GPa, friction coefficient below 0.3 at temperatures above 600 ◦C, and heat resistance up to 800 ◦C [23]. The introduction of Si3N4 into the TaSi2 coatings allows for a substantial increase in oxidation resistance [24]. The Ta-Si-N coatings have high thermal stability up to 900 ◦C [25]. Moreover, Ta-Si-N coatings possess good diffusion barrier properties [26] and are used to prolongate the lifespan of high-speed steel cutting tools [27] and glass forms [28]. Increased Si content endows the Ta-Si-N coatings with oxidation resistance up to 1300 ◦C [29]. A similar effect is produced by boron alloying, as it promotes the formation of borosilicate glass in the oxide layer, thereby reducing the viscosity and increasing the protective properties of the oxide [30].

The aim of this work is the comparative study of the structures and properties of Ta-Zr-Si-B-(C,N) coatings deposited by magnetron sputtering of composite target TaSi2-Ta3B4-(Ta,Zr)B2 in inert (Ar) and reactive atmospheres (Ar-N2 and Ar-C2H4).
